This invention relates to blow-molded articles, and more particularly to convoluted boots and the like formed from relatively hard polymer materials, as compared to the softer rubbers, for use with constant velocity joints.
Constant velocity or CV joints, as used on automotive drive axles, such as on front wheel drive cars, are commonly enclosed within a convoluted elastomer boot. Such convoluted boot, that is, boots having a plurality of axially spaced annular convolutions, are commonly formed with a small end which is retained by a clamp on the drive shaft leading out of the joint, and a large end clamped to an annular surface of the body of the CV joint. Such boots provide a seal for the joint over the operating range of the constant velocity joint, which may include drive offset angles from between 0.degree. up to 40.degree. or more between the shaft and the joint body. Such boots are commonly molded or rubber, which may have a hardness in the range of Shore 65A, for example, which permits the boot to flex with the flexing of the joint.
More recently, blow molded boots formed of more rigid thermoplastic polyester elastomers have come into use as a substitute for the rubber boot, by reason of the greater mechanical and physical strength of such elastomers over a wide variety of operating conditions, and generally by reason of the greater toughness of such elastomers and their ability to resist puncture or tearing. However, such blow molded polymers are considerably harder and stiffer than their rubber counterparts, and may, for example, have a hardness in the range of Shore 50D and a flexural modulus of 18,009 psi or more (at 22.degree. C.).
Typical materials from which blow-molded convoluted boots have been made are commonly referred to as thermoplastic elastomers (TPE) and include polyester, polyether, polybutylterephthalate compounds (PEEBT). Typical thermoplastic elastomer materials which are used for blow-molding convoluted boots, as defined above, include du Pont "Hytrel", HTG-5612 and Monsanto's "Santoprene" thermoplastic rubber, typically grade 103-40. Such elastomers, as compared to typical rubbers, have a substantially greater tendency to deform at relatively low elongation forces, and typically have a maximum or 100% modulus which is less than half of the ultimate tensile stress.
Such blow-molding elastomers have limited ability to accept tensile loading without undergoing deformation. Accordingly, it has been found necessary in simple convoluted boots, to mold the same at their maximum extension or length as expected in use, so that the boots will experience only compression, and also will not be subject to buckling when axially extended to the maximum.
The requirements for maintaining the material under relatively low tension, or no tension at all, are particularly difficult to obtain when the boot is designed to operate in conjunction with a constant velocity joint, where the joint may, from time to time, be operated at relatively high angular offsets. Such high angular offsets can introduce an undesired tension into the boot material resulting in premature failure. In addition, the problem of anchoring such boots at the sleeve-like ends is complicated by the relative hardness of the material and the stresses applied to the clamping areas.